The susceptibility of starter cultures to bacteriophage infection remains a problem in the cheese industry, especially with increasing reliance on defined starters and the high turnover in factories. Analysis of natural phage resistance mechanisms in starters has led to the identification of four categories of bacteriophage resistance in lactococci: interference with either 1) phage adsorption, or 2) phage DNA injection; 3) DNA restriction/modification (R/M); and 4) abortive infection (Abi) via a disruption of phage development post infection. Numerous plasmids encoding one or more of these systems have been isolated from commercial starter strains.
Lactococcus lactis ME2 is a prototype phage-resistant strain which has been used successfully in the cheese industry. ME2 contains at least three plasmids which encode distinct phage defenses. Plasmid pME0030 interferes with phage adsorption. Two self-transmissible plasmids, pTR2030 and pTN20, each encode at least one R/M system and one Abi system.
Plasmid pTR2030 has previously been partially subcloned and sequenced to reveal an R/M system, LlaI, and abortive gene, abiA.